Imagine Janeway reclaiming a cargo bay to turn it into a hydroponics bay - even a ship the size of the Defiant must have a cargo bay - at the expense of cargo space
Defiant had one cargo bay according to dialogue in the 5th season.
forcing the surviving Starfleet crew to bunk up with members of the Maquis (instead of seemingly giving everyone their own rooms) which would be conducive to the Kim/Paris friendship and put an interesting spin on the inclusion of Neelix and Kes, who would have either had to bunk with other crew (Tuvok with Neelix would have been interesting, Kes presumably with either Samantha Wildman or B'Elanna).
They would've had to do that anyway if a Defiant had been used.
Voyager actually already having a kitchen always felt like a bit of a cop-out in retrospect, imagine them having to find the space to build a kitchen
Voyager didn't have a kitchen when it left spacedock. The kitchen was built by Neelix without authorization (In Caretaker the spot where Neelix's kitchen would soon be had a large bank of Replicators).